The present invention concerns recovery of digital sequences transmitted over a band-limited and dispersive radio channel. More specifically, it relates to employing both diversity reception and maximum-likelihood-sequence estimation for this purpose simultaneously.
Digital signals transmitted over a band-limited and dispersive radio channel are likely to experience noise corruption, intersymbol-interference (ISI) distortion, and multipath fading. A significant part of these effects results from a time-varying channel between the transmitter and receiver, and numerous techniques have been employed to reduce the less-desirable results of such time-varying channels. Two of those, namely, diversity reception and sequence estimation, are of particular interest here.
Diversity reception has been used for some time to reduce the effects of fading channels. In a time-varying channel, the signal from a given source can vary over time from a strong, clear signal to one that is so weak as to be buried in noise. Reliability of communication with such a source can be improved by employing multiple independent channels so that it is unlikely that all will be too weak at any one time to produce good results. Diversity can be provided, for instance, by using multiple antennas that are located and/or oriented differently. One can then choose the channel that currently is providing the greatest output power, for instance, or one can combine the various outputs, possibly by using weighting factors that depend on the various channels' output powers.
The sequence-estimation technique is exemplified, for instance, by the maximum-likelihood-sequence-estimation (MLSE) algorithm originally proposed in Forney, "Maximum-Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference," IEEE Trans. Inform. Theory, vol. IT-18, pp. 363-78 (May 1972). An adaptive version of the MLSE algorithm was proposed by F. R. Magee and J. G. Proakis with reference to Proakis, Digital Communications (McGraw-Hill 1989). A simpler, sub-optimal approach referred to as "reduced-state sequence estimation" (RSSE) was described by Eyuboglu and Qureshi in "Reduced-State Sequence Estimation with Set Partitioning and Decision Feedback," IEEE Trans. Comm., vol. COM-36, pp. 13-20 (January 1988).
The general approach employed in sequence estimation involves maintaining a model of the (typically time-varying) channel and applying to that model all of the sequences of symbols that the employed communications protocol permits. By computing "metrics" representing the differences between actually received signals and the model's responses to each hypothetical sequence, the sequence-estimation technique determines which sequence is the one most likely to have been transmitted.
In the adaptive sequence-estimation approaches, which are those typically employed for time-varying channels, the model is updated by comparing the received signal with the response of the model to a reference symbol sequence known to have been transmitted. Initially, a channel-impulse-response estimator, which maintains the model, is "trained" by using as a reference a predetermined sequence known to be transmitted during, for instance, certain "header" periods dictated by the communicative system's protocol.
Once the estimator has been "trained," some systems continue to adapt the model by "tracking." In the tracking mode, the channel-impulse-response estimator does not use a predetermined sequence as its reference. It instead uses the sequence that the sequence-estimation algorithm determines to have been sent. That is, it applies the thus-determined sequence to the currently prevailing model and compares the resultant output with (a delayed version of) the received signal that resulted in the determined receiver output. It continually adjusts the model in accordance with the difference between the received signal and the model output.
U.S. Pat. No. 5,031,193 to Atkinson et al. proposes the use of both diversity reception and sequence estimation in the same receiver. In the system described there, a separate set of equalizer tap gains is adaptively maintained for each of a plurality of diversity channels. Specifically, an equalizer in each channel filters the received signal in accordance with the tap gains maintained for that channel and thus removes the channel's distorting effects to some extent. In particular, it reduces phase differences among the channels. The equalized outputs of all channels are then applied to a diversity device, such as a device for computing the weighted sum of those equalized outputs, and the result is applied to a decision device, which performs some type of sequence estimation. Concurrently, a decision circuit in each channel determines from the resultant, equalized signal the symbols that must have been sent on that channel, and an adjustment algorithm compares the equalizer's output with these symbols and updates the parameters accordingly.